After spending 12 lonely years traversing the solar system, the Rosetta spacecraft is set to send its last transmissions home — before crash-landing onto the surface of a comet more than 700 million kilometres away from Earth.

The spacecraft, which is the size of a mini-bus with wings, was launched in 2004 on a mission to determine whether comets contain the building blocks of life.

For the last two years, it has been circling a comet called 67P/Churyumov-Gerasimenko as the comet travelled in towards the Sun.

In this time, Rosetta has dropped a probe onto the comet and travelled around the Sun sending back data about the comet's surface and chemistry.

Now, as the comet moves further away from the Sun again, Rosetta has less solar power, making it difficult for the craft to operate its instruments.

With diminishing scientific returns and the extended costs of trying to keep the spacecraft flying, scientists at the European Space Agency (ESA) decided to squeeze every last drop of science from the mission.

Rosetta's final act will be to send data and close-up images on its 14-hour journey to a specific landing point on the comet's surface.

A crash landing in slow motion

With the craft moving close to walking speed this will be a very controlled crash, said Warwick Holmes, a former ESA engineer who helped build Rosetta.

The descent will begin around 6:50AM AEST on Friday (22:00 UTC Thursday September 29) when scientists tell the craft to fire some of its 24 thrusters for three minutes, bringing its orbit to a halt.

From that point, the craft will be in a 19 kilometre freefall as it heads towards the surface of comet 67P in ultra slow-motion.

Rosetta will free-fall for 19 kilometres before colliding with comet 67P

Supplied: ESA

"About two kilometres above the surface we're going 0.6 metres per second. Then at touchdown we're doing 0.9 metres per second, which is about three kilometres an hour," Mr Holmes said.

He said the ESA team will know that Rosetta has collided with the comet because the signal it's transmitting home will suddenly disappear as the high-gain antenna onboard the craft is jolted away from Earth.

"When we crash, that high gain antenna will jolt massively, and it will just lose the Earth's transmission," he said.

"There's no electronics that could then re-acquire the signal."

On a collision course with the Ma'at region

The scientists have calculated Rosetta will land on the Ma'at region on the head of the duck-shaped comet.

As it descends to the planned landing point, Rosetta will take measurements of the dust, plasma, and gas in the environment close to the surface, as well as high-resolution photographs of mysterious pit holes in the comet.

The Ma'at region is of particular interest to scientists because those pit holes haven't been seen anywhere else in our solar system.

Some pits in the area run up to 60 metres deep and might allow scientists to peer inside of the comet, giving clues as to how comets form.

"They don't exist on any other planetary body in our system, we've never seen them before," Mr Holmes said.

"We're going to be seeing millimetre-level resolution from these very powerful Osiris cameras that we have on Rosetta."

Rosetta will land in the Ma'at region on the head of the comet

(Supplied: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Supplied: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Exactly what is revealed will depend on the images captured by Rosetta in its last moments.

Scientists still aren't sure exactly how the gravity of the comet close-by to the surface will affect the spacecraft, and they're also concerned about how dust and gas spewing from the comet might affect its trajectory.

But the ESA is predicting the craft will land relatively close to the point on the comet it's aiming at despite these uncertainties.

Rosetta an 'ark of humanity'

Despite the low gravity of the comet, Rosetta should remain on its surface for billions of years if nothing disturbs it.

Mr Holmes said the craft is a veritable "ark of humanity," because its insides contained the human DNA of all those who had worked on assembling it.

"Humans, myself included, spent dozens of hours deep inside the spacecraft doing all the electrical connections and the electrical testing ... I know many times I was sneezing or coughing," he said.

He hoped Rosetta might one day be recovered and opened up to find the preserved DNA of the people who worked on the craft.

"The decision's now been made to put Rosetta down onto the surface, and as a consequence it's become a Noah's ark of human DNA, for aliens or future humans to go up and clone us all over again."